Devices for measuring the earth's surface vibration are known as "geophones." A geophone may be considered a special application of devices used for measuring vibration, which are sometimes called "vibrometers." Movement that is not vibration may also be measured by such devices. The word "geophone" used herein will be understood to include an instrument used for measuring movement or vibration of a surface.
Geophones are used for seismic exploration both on land and under water. They are contained in a case that contacts the surface of the earth and usually connects to electrical cables. The principle of the conventional geophone is very simple: a coil of electrical wire is suspended from a spring in the center of permanent magnets. The mass of the coil, suspended from the spring, tends to remain stationary when the case moves up and down. This causes the electrical coil to move through the magnetic field of the permanent magnets and induces small electrical currents in the coil. Electrical voltage produced is proportional to the velocity of motion. The voltage is recorded in a computer memory or other memory device for later analysis. Geophones based on this principle have been plagued by limited data quality. The quality of the data gathered has been limited because the reference body is not fully de-coupled from the vibration; it is weakly coupled to the apparatus by the spring. Therefore, the reference body is influenced by the motion that is to be measured, which complicates analysis of the measured data. The response of the reference to the surface motion must compensated for.
Resonant frequency is one of the important characteristics of a conventional geophone. Lower resonant frequency is desired but is more difficult to achieve. Conventional geophones commonly have a resonant frequency in the range from 1 Hz to about 10 Hz.
The response of a conventional geophone is measured in volts per g of acceleration and varies with frequency of the movement. Sensitivity is limited by the noise of the electronics and the low frequency response (less than 5 Hz) is inadequate for many applications. Geophones for use in seismic exploration should have low distortion, large dynamic range, be insensitive to off-axis inputs, survive shock inputs of greater than 10,000 g's with no effect on performance and have a wide operating temperature range.
Seismometers using either mass velocity or displacement feedback have been described in the literature and are manufactured by various companies. The newer micromachined accelerometers are generally robust, but are typically expensive and do not offer equivalent performance. The sensitivity of geophones that are commonly available depends on whether a closed loop or open loop method of measurement is used, but commonly is in the range of a few volts per g. In the closed loop configuration, sensitivity can be fairly flat from less than 1 Hz to about 100 Hz. Sensitivity above about 100 Hz drops rapidly. The spring and mass system is often tuned to a resonant frequency of about 7 Hz in conventional geophones. Because of a low spring constant, a large amount of damping must be applied to the system. The damping causes loss of signal. The lower sensitivity leads to loss of bandwidth.
Because of the accuracy limitations of current geophone technology, the required accuracy is achieved in the field by connecting several geophones in series. Each such grouping of geophones is called a string. For analysis purposes, each string is taken to approximate a point measurement despite the fact that the string is, in reality, an extended body.
A laser geophone was disclosed in U.S. Pat. No. 4,284,350. The method involves directing a light beam from a laser to a location, reflecting the beam from a reflector that moves with surface motion of the earth at that location and also, at that location, reflecting the beam from a reflector that is partially isolated from motion of the earth by a spring. Movement of the reflectors is measured by the Doppler shift of frequency.
A laser interferometer for use in a geophone was disclosed in U.S. Pat. No. 4,500,979. A mass that is suspended by a spring has top and bottom reflectors that reflect a beam that is combined with a reference beam from the laser to produce interference fringe patterns in response to movement of the mass. Counting of fringes and fractions of fringes in a unit of time is used to measure velocity of the mass.
While improvements in conventional geophone apparatus have occurred steadily for many years, the sensitivity and frequency response of such geophones are limited. To obtain a transducer having properties not limited by the prior art mechanisms, a different principle for measuring motion or velocity is needed. The technique should allow for measuring small displacements independent of frequency over a broad range of frequencies. Importantly, coupling between the earth's surface and a reference body should be eliminated.